1. Field of the Invention
The present invention relates to a magneto-optical recording method for recording data with a large capacity by increasing a recording density, and an apparatus to be used in the recording method.
2. Description of Related Art
Generally, information is recorded in a magneto-optical disk in the following manner: A laser beam is converged by a lens, so as to change its intensity in accordance with the information to be recorded. The magnetization direction in an area irradiated with the laser beam is changed by applying an external magnetic field, thereby forming an inverted magnetic domain in which the information is recorded. The information recorded in the magneto-optical disk is reproduced by irradiating the surface of the disk with a laser beam weaker than that used for recording the information so as to detect the rotation of a plane of polarization caused by a difference in the magnetization direction on the disk.
Recently, there are increasing demands for a larger capacity and a higher recording density of such a magneto-optical disk. Various methods for increasing the capacity and density of the magneto-optical disk are known, such as a method for increasing a recording track density by decreasing a track pitch; a mark edge recording method in which information is included in an edge portion of an inverted magnetic domain; a land/groove recording method in which recording marks are formed in respective land and groove portions; and a method using an MSR (magnetic super-resolution) medium which is recently developed as a novel magneto-optical disk.
In the mark edge recording method among these known methods, the front and back edges of a recording mark correspond to information of "1". This method is set against a pit position recording method in which the presence and the absence of a recording mark respectively correspond to information of "1" and "0". In comparison with the pit position recording method, the mark edge recording method can attain remarkably high density recording.
The mark edge recording method is thus effective in high density recording, but it is necessary to form a recording mark, that is, an inverted magnetic domain, in a position and a length accurately according to data to be recorded. Therefore, power control of a used beam such as a laser beam and timing control of beam irradiation are significant factors in this method.
When a power of a laser beam, that is, a beam power, is simply changed in accordance with data to be recorded without taking a thermal effect of the laser beam on a recording face of the magneto-optical disk into consideration, the relationship among the data to be recorded, the beam power, temperature distribution and resultant recording marks is obtained as is shown in FIG. 1.
FIG. 1 is a waveform diagram showing the relationship among the data to be recorded, the beam power adopted for recording, the temperature distribution on the recording face of the magneto-optical disk and the recording marks formed on the recording face of the magneto-optical disk obtained by a conventional magneto-optical recording method.
As is obvious from FIG. 1, in accordance with the data to be recorded, the beam power rises from a bottom power P.sub.b to a writing power P.sub.w simultaneously with the rise of the data, is retained at a high level while the data is being at a high level, and falls from the writing power P.sub.w to the bottom power P.sub.b simultaneously with the fall of the data. Thus, the beam power is shown as a rectangular waveform.
As a result, the temperature distribution on the recording face of the magneto-optical disk increases with time. When a long recording mark is to be formed, the recording mark is formed in the shape of a teardrop and the back edge of the mark is shifted as shown in FIG. 1 due to stored heat. When a space between adjacent recording marks is small, the front edge of the latter mark is shifted due to heat interference caused by the former mark.
As a countermeasure against this problem, Japanese Patent Application Laid-Open No. 5-290437 (1993) discloses a multi-pulse recording method for suppressing the shift caused by such stored heat and heat interference.
This publication describes a technique for changing the power of a laser beam in three stages of a writing power P.sub.w, an assist power P.sub.a for warm-up and a bottom power P.sub.b which is the lowest in a data recording operation on a magneto-optical disk.
FIG. 2 is a waveform diagram for showing the relationship among the data to be recorded, the beam power, the temperature distribution on the recording face of the magneto-optical disk and resultant recording marks obtained by this technique.
As is shown in FIG. 2, in an area where a recording mark is to be formed (hereinafter referred to as a recording mark area), the beam power rises from the assist power P.sub.a to the writing power P.sub.w simultaneously with the rise of the data to be recorded, then is retained at this level for a predetermined period of time, and falls from the writing power P.sub.w to the assist power P.sub.a. Thereafter, the beam is emitted in the form of a pulse train in which the assist power P.sub.a and the writing power P.sub.w are alternately repeated in accordance with a length of the data to be recorded.
Prior to the fall of the data, the beam power falls from the writing power P.sub.w to the bottom power P.sub.b, and is retained at this level until the fall of the data. After irradiating an area between the recording marks where a space is to be formed (hereinafter referred to as a space area) for a predetermined period of time, the beam power returns from the bottom power P.sub.b to the assist power P.sub.a.
In the space area, the beam power is retained at the assist power P.sub.a, and the aforementioned pattern is repeated correspondingly to the rise of subsequent data. As a result, the temperature distribution on the magneto-optical disk substantially corresponds to the level of the data, i.e. high or low, as is shown in FIG. 2, and the recording marks are formed in lengths in accordance with the data to be recorded as is shown in FIG. 2.
In this manner, in the recording mark area, the beam is emitted in a pulse-like manner in which the writing power P.sub.w and the assist power P.sub.a are alternately repeated, thereby suppressing heat from being stored on the magneto-optical disk. Around the back edge of the recording mark area, the beam power is changed to the bottom power P.sub.b, thereby conducting a heat cut-off process. In the space area, the beam power is returned to the assist power P.sub.a and retained at this level, so that a temperature change due to the heat interference caused by forming the previous recording mark is balanced with warm-up temperature increase caused by the assist power P.sub.a. Thus, the temperature distribution in the space area is made constant.
As a result, regardless of the length of the space area, the temperature at the front edge of the latter recording mark is made constant, thereby suppressing the front edge of this recording mark from being shifted by the heat interference.
In such a conventional multi-pulse recording method, the shift of the edge of a recording mark can be suppressed as compared with the method in which the beam power is changed in the rectangular waveform as shown in FIG. 1. However, when a recorded pattern is actually reproduced, so-called jitter, which indicates the degree of variation in the position of an edge with respect to a reproducing clock as a basis of a reproducing signal, is disadvantageously large.
The jitter becomes large for the following reason:
The jitter is considered to be caused by various kinds of noises such as a laser diode (LD) noise, a noise due to the variation in the sensitivity of a disk and a circuit noise. A mechanism of jitter occurrence in data recording is generally considered as follows:
In either of a recording mark area and a space area, the beam is continuously emitted with the assist power P.sub.a for warm-up, and hence the temperature at the recording face of the magneto-optical disk is increased. As a result, a difference t between the writing power P.sub.w and the assist power P.sub.a becomes small, so as to make gentle the temperature change at the front edge of the recording mark.
When the temperature change becomes gentle, the variation in the temperature distribution due to the variation in the sensitivity of the magneto-optical disk is affected, thereby disturbing the position of the front edge of the recording mark and enlarging the jitter. In addition, the jitter is likely to be increased as the recording mark is smaller. This has been a serious obstacle to high density recording.